(a) Field of the Invention
The present invention relates to an optical fiber bundle and, more particularly, to an optical fiber bundle for which respective fibers thereof are firmly fixed together at its end portions and, at the same time, are separated from each other at the middle portion other than end portions so that said optical fiber bundle will have flexibility.
(B0 Descipton of the Prior Art
For an image guide or the like used, for example, for endoscope, it is necessary to fix perspective fibers together, at both end portions of the image fiber, so that respective fibers will be kept in identical geometrical patterns at both end portions in order to transmit the image focused on one end face of the image guide to the opposite end face so that a clear-cut and correct image can be obtained on said opposite end face. Besides, it is necessary to arrange the endoscope so that it can be inserted to any desired portion in order to make it possible to observe any such portion. Therefore, the image fiber to be used for the endoscope should have, as far as possible, high flexibility at its middle portion other than its both end portions. Consequently, for the image fiber, it is so required that respective fibers are kept separated from each other at its middle portion.
To meet the above-mentioned requirement, optical fiber bundles prepared by a method as described below are known. That is, as shown in FIG. 1, an optical fiber 4 is prepared by providing a layer 3 of acid-soluble glass on the outer surface of known optical fiber assembly consisting of a light-conducting core 1 made of glass having a comparatively high refractive index and cladding 2 made of glass having a comparatively low refractive index and provided on the outer surface of the core 1. Many of said optical fibers 4 are bundled together, and that optical fiber bundle is inserted into a cylindrical tube 5 made of acid-soluble glass and is drawn by heating by a heater 6 as shown in FIG. 2. Thus, a rod-type fiber bundle 7 is formed. After covering both end portions of said rod-type fiber bundle 7 with acid-resisting material, the rod-type fiber bundle 7 is immersed in acid in order to dissolve and remove the cylindrical tube 5 of acid-soluble glass and the layer 3 of acid-soluble glass covering the middle portion of each fiber 4 except for those covering both end portions. Thus, respective fibers are separated from each other at the middle portion and a flexible optical fiber bundle is obtained. The optical fiber bundle 7 made by the method as described in the above has, as shown in FIG. 3, both end portions 7a and 7b at which respective fibers 4 are fixed together and the middle portion 7c at which respective fibers are separated from each other. For the most part of the middle portion 7c at which respective fibers 4 are separated from each other, acid-soluble glass layers 3 are dissolved by acid and removed completely. However, at portions adjacent to border lines between the middle portion 7c where respective fibers are separated from each other and both end portions 7a and 7b, sludge 8 of glass which is not completely dissolved by the acid remains unremoved because the acid does not sufficiently permeate into interstices between respective fibers as such portions. (Refer to FIG. 4.) The amount of such sludge 8 becomes larger at portions closer to said border lines. (When glass containing SiO.sub.2 is used as said acid-soluble glass, said sludge 8 also contains SiO.sub.2, etc. which are comparatively insoluble in acid.) To eliminate such sludge 8 which remains unremoved, it is preferable to make the acid-soluble glass layer 3 provided to each fiber 4 thicker. When, however, the layer 3 is made thicker, the ratio of the portion which is not useful for light conducting becomes larger and, therefore, the light-conducting efficiency decreases. In practice, the layer 3 is therefore made as thin as possible and its thickness is about 1 .mu. to 5 .mu. in general. Consequently, it is impossible to eliminate the above-mentioned sludge 8 completely.
As described in the above, respective fibers of this kind of optical fiber bundle are fixed together at their both end portions 7a and 7b and cannot be moved at all. On the other hand, at those portions of the middle portion 7c which can close to the above-mentioned border lines, respective fibers can be moved as the acid-soluble glass is partially removed though their movement is somewhat restricted because the above-mentioned sludge 8 exists. Moreover, this kind of optical fiber bundle is frequently bent when using as described already. When the optical fiber bundle is bent, stress is concentratively applied to portions near the above-mentioned border lines between the separated portion 7c and fixed portions 7a and 7b. Consequently, respective optical fibers have a strong tendency to be broken at those portions near the border lines. Even if the stress applied to portions close to said border lines is not so large to cause breakage to fibers, respective fibers are injured, when they are bent, because of the sludge such as acid-soluble glass which was not dissolved completely, SiO.sub.2, etc., in case such sludge exists between respective fibers and, therefore, respective fibers become susceptible to breakage. Moreover, if said sludge exists as lumps between respective fibers, the bent fibers are subjected to an extremely large concentrated stress at the position where such lump of sludge exists and, therefore, tend to be broken.